The Artemis Racing crash was the second involving the spectacular new AC72 size of sailing boat, after one involving the Oracle team in October, in which no one was injured, and has revived questions about the new boat

Given the circumstances, the Oracle team would not train with its two AC72s before Monday, said America's Cup Regatta Director Iain Murray. Two other teams with AC72s are not yet ready to train.

The San Francisco Police Department will lead its own probe, along with coastguards, to "rule out any criminal negligence or intent," said captain Matt Bliven of the US Coast Guard.

Yesterday I lost my closest friend of over twenty five years, the friendliest and kindest man I have ever met. I cannot believe he is no longer with us.

British Olympian Iain Percy

Thursday's accident happened during regular training and in normal conditions in San Francisco Bay, with flat water and winds of 18-20 knots, organizers told a news conference.

"The boat nose-dived... the boat ended up upside-down capsized, broken into many pieces," said Murray, who struggled to compose himself at first. He said Simpson, known by his nickname "Bart" was "a mate."

"It appears Bart was trapped under the solid sections of the yacht out of view, out of sight to the myriad people on board trying to locate him, including divers with proper apparatus," Murray added.

"All the crews had been trained under water, they all carried oxygen and were prepared for the worst. Andrew was located eventually" and doctors on support boats and later on the docks tried to revive him.

"Unfortunately they were unable to retrieve him and he passed," he said, adding that another sailor, Olympic bronze medallist Craig Monk of New Zealand, suffered minor injuries.

The AC72 boats developed for the 2013 America's Cup are described by event organizers as "speedsters" powered by 130-foot (39.6-meter) tall wing sails and with the ability to hydrofoil, rising out of the water to reduce drag.

They can reach top speeds in excess of twice the wind speed, and place strenuous physical demands on crew members.

A huge loss

Simpson was an experienced yachtsman, winner of Olympic gold in Beijing in 2008 and Olympic silver in 2012 in the keelboat star class.

He had moved to San Francisco with wife Leah and sons Freddie and Hamish to train for six months.

Yesterday, his British Olympic teammate Iain Percy paid tribute to the 36-year-old.

"Yesterday I lost my closest friend of over twenty five years, the friendliest and kindest man I have ever met," Percy said.

"I cannot believe he is no longer with us. Now all our thoughts should be with his wife and two amazing boys who meant the world to him.

"Andrew has more friends than anyone and we will continue to support his family with all our hearts."

Britain's Ben Ainslie, the most successful Olympic sailor, joined the sailing community in mourning Simpson's death.

"This is such a tragedy," Ainslie said. "Andrew was such a wonderful husband, father, friend and one of the nicest people you would ever meet.

"The only solace I can find is that he died doing something which he loved.

"I have such fond memories of growing up sailing together as kids and then as adults. I will miss him so much."